1. Field of the Invention
This invention relates to high-Q micromechanical devices such as capacitors and capacitive switches and methods of tuning same.
2. Background Art
Micromechanical tunable capacitors constructed using MEMS technology have previously been demonstrated with Q""s on the order of 60xe2x80x94a value that greatly exceeds those achievable by semiconductor diode counterparts fabricated via conventional IC technology. Such micromechanical capacitors often consist of suspended top metal plates that can be electrostatically displaced (via applied voltages) over bottom metal plates to vary the capacitance between the plates. Because these capacitors can be constructed in low resistivity metal materials, they exhibit much larger Q""s that their semiconductor diode counterparts, which suffer from greater losses due to excessive semiconductor series resistance. To date, micromechanical capacitors have been successfully applied toward the implementation of on-chip, high-Q LC tanks for use in low-phase noise, communications-grade voltage-controlled oscillators (VCO""s).
Recent advances in micromechanical tunable capacitor technology, however, are beginning to extend the application range of such devices beyond the initial focus on LC tanks for VCO""s, toward the new challenge of tunable preselect filters for multi-band reconfigurable wireless communication handsets. For this application, much higher Q""s are required, on the order of 200 or more. Despite the use of metal in their construction, the Q of micromechanical capacitors to date is still limited by losses arising from the finite resistivity of their metal suspension beams, which often must be made long to attain stiffness values low enough to insure sufficiently low actuation voltages. In effect, traditional micromechanical capacitor designs clearly exhibit a Q versus actuation voltage trade-off.
The U.S. patent to Bauhahn, 5,696,662, discloses an electrostatically-operated micromechanical capacitor which is tunable by moving pairs of plates linearly relative to each other through the application of a voltage to the plates.
The U.S. patents to Chang et al., U.S. Pat. Nos. 5,959,516 and 6,094,102, disclose a high-Q MEMS capacitor wherein a central voltage applied to a master or central capacitor sets the capacitance of a slave or signal capacitor.
An object of the present invention is to provide a high-Q micromechanical device such as a capacitor and method of tuning same which break the above trade-off by eliminating the need for lengthy top plate suspension beams. Specifically, rather than implement tunability using a movable top plate, the top plate is made stationary, and the dielectric between the metal plates is made movable. In effect, capacitive tuning is attained via a tunable-dielectric, realized via a movable dielectric plate suspended by dielectric beams that do not impact the Q of the device, and hence, allow Q""s of up to 290 and above.
In carrying out the above object and other objects of the present invention, a high-Q micromechanical device such as a capacitor is provided. The capacitor includes a substrate, a pair of conductive layers supported on the substrate and having a capacitive gap therebetween, and a dielectric disposed in the gap between the conductive layers. The capacitor also includes means for displacing the dielectric within the gap between the conductive layers to tune the capacitor over a tuning range.
The means for displacing may electrostatically displace the dielectric in the gap.
The capacitor may further include at least one spring element coupled to the dielectric to move the dielectric between the layers.
The at least one spring element may include a lateral or vertical spring element supported on the substrate.
One of the conductive layers may form at least a portion of a top plate and the other conductive layer may form at least a portion of a bottom plate wherein both of the top and bottom plates are fixed to the substrate.
Each of the conductive layers may be a conductive metal.
The substrate may be a semiconductor substrate.
The tuning range may be based on a ratio of thickness of the dielectric to thickness of the gap between the conductive layers.
A Q factor of the capacitor may be greater than 50, or 200 or even 290.
The device may be a capacitive switch.
Further in carrying out the above object and other objects of the present invention, a method for tuning a micromechanical device such as a capacitor is provided. The method includes providing a pair of conductive layers supported on a substrate and having a capacitive gap therebetween, and providing a dielectric in the gap between the conductive layers. The method further includes applying a voltage bias to the conductive layers to electrostatically displace the dielectric between the conductive layers.
The method may further include moving the dielectric between the layers.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.